The Quasars' New Clothes

The more we learn, the less we seem to know about quasars.

Astronomers have been itching for more than 30 years to get a good look at quasars. At first everyone thought these peculiar objects were just odd-looking stars in the Milky Way. But the realization that quasars were really out at the edge of the observable universe, and thus must be far brighter than the brightest galaxy, posed a riddle that nobody has yet been able to answer with certainty: What are they?

Everyone knew that the Hubble telescope might provide some answers--and everyone was thus eager to hear John Bahcall’s talk at the American Astronomical Society’s annual winter meeting last January. The eminent astronomer from the Institute for Advanced Study at Princeton stepped up to the podium, announced that he and two collaborators had taken high-resolution Hubble images of 15 quasars, and proclaimed the result: We have taken a giant leap backward in our understanding of quasars.

The general (if unproven) understanding of quasars has been that they result from gas trying to cram its way down the gravitational throats of supermassive black holes. The gas, compressed and heated to millions of degrees, glows brightly enough to be seen for billions of light-years. Because such black holes are most likely to exist at the cores of galaxies, a close enough look at a quasar should usually reveal the host around it. But in 11 of Bahcall’s 15 images, the quasars seemed to stand alone--naked, without host galaxies to shelter and fuel them.

At first all those naked quasars had astronomers buzzing. Some even toyed with throwing the whole black-hole model out. By fall, though, Bahcall and his colleagues were leaning toward a less radical interpretation. We’ve now got observations of 20 quasars, says Bahcall. And while the original naked quasars are still naked, he says, many of the new ones do show evidence--in some cases, quite dramatic evidence--of very normal-looking host galaxies. It may be that the others have host galaxies, too, and that we simply haven’t looked hard enough yet.

That in itself is still a surprise. Theorists have always thought that quasars should form most easily in bright galaxies, rich in gas and young stars, rather than in fainter and more normal ones. Bahcall’s observations may not be a giant leap backward for quasar mavens--but they are certainly a significant hop sideways.